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ameckO <br /> Mr. Howard Hold <br /> California Regional Water Quality Control Board <br /> Central Valley Region <br /> April 27, 2010 <br /> Page 7 <br /> In summary, the Cr and Cr VI groundwater concentrations are below the CDPH MCL of 50 pg/L <br /> and exhibit a stable to decreasing trend. <br /> SUMMARY <br /> The following list summarizes the results from the three groundwater monitoring events <br /> conducted at the Lathrop Facility. <br /> • Groundwater elevations measured during this period ranged from 1.91 feet bmsl to <br /> 3.25 feet amsl. The calculated groundwater elevations were within the range of <br /> historical elevations calculated for the Lathrop Facility. <br /> • The groundwater gradients calculated during this period ranged between 0.004 to <br /> 0.009 ft/ft, with a flow direction to the northeast, similar to historical gradients and <br /> flow directions for the Lathrop Facility. <br /> • For the three monitoring events, TPH-diesel was not detected above the laboratory <br /> reporting limit of 50 pg/L in the groundwater samples from all three monitoring wells <br /> (LPW-1, LPW-2, and LPW-3), which is consistent with the November 2007, May and <br /> November 2008, and May and November 2009 monitoring events at the Lathrop <br /> Facility. <br /> • For the three monitoring events, total and dissolved Cr concentrations were detected <br /> below the CDPH MCL (50 pg/L) in the groundwater samples collected from <br /> monitoring wells LPW-1 and LPW-2. The groundwater sample concentrations of <br /> total Cr ranged between 3.7J and 14 pg/L, and dissolved Cr ranged between 3.5J <br /> and 12 pg/L. Total and dissolved Cr were not detected above the laboratory <br /> reporting limit (10 pg/L) in the groundwater samples from well LPW-3. Groundwater <br /> sample results for total and dissolved Cr for the three sampling events were within <br /> the range of concentrations detected during previous events. <br /> • For the three monitoring events, dissolved Cr VI concentrations were detected below <br /> the CDPH MCL (50 pg/L) in the groundwater samples collected from all three <br /> monitoring wells at concentrations ranging between 2.8 and 14 ug/L. <br /> • Since the removal of the stockpile in October 2007, no metals were detected in the <br /> groundwater samples above the CDPH MCL of 50 pg/L for Cr and the prevailing <br /> trend of groundwater sample concentrations at wells LPW-1, LPW-2, and LPW-3 <br /> appears to be stable or decreasing (Figures 6 through 11). <br /> P:\10626.000.0\Docs\Semi-Annual Groundwater Monitoring&Sampling\Lathrop Monthly Groundwater Monitoring\Ltr_042710.doc <br />